RNA interference (RNAi) has attracted massive attention in recent years, as it provides a means to silence the expression of a target gene. It provides basic research with a method for studying genetic and biochemical pathways, and the function of individual genes and gene products. Consequently, RNA interference has become a critical tool for target validation in the pharmaceutical industry. Moreover, substantial investments have been made with the goal of developing RNA complexes capable of mediating RNA interference complexes that can be used as drugs.
The attractiveness of RNAi for use in therapy lies in its sensitivity and sequence specificity. However, concerns have arisen concerning sequence specificity, e.g. because the wrong strand of the RNA complex may direct the response to the wrong target nucleic acids. Moreover, RNA complexes of a certain size induce a non-specific interferon dependent response, which is also undesirable.
Patent application US2003/0108923 describes RNA complexes capable of mediating RNAi comprising an antisense strand and a passenger strand, wherein the strands are 21-23 nucleotides in length. It is suggested that the RNA complexes are used for therapeutic applications.
Similarly, patent application US2005/0234007 describes RNA complexes capable of mediating RNAi comprising an antisense strand and a passenger strand, wherein the complex comprises 3′-overhangs. It is suggested that the RNA complexes are used for therapeutic applications.
WO2005/073378 describes RNAi complexes containing chemically modified nucleotides capable of mediating RNAi comprising an antisense strand and a passenger strand. The RNA complexes described in the specification comprise LNA residues and it is stated that incorporation of LNA residues near the 5′end of one of the strand can control which strand is incorporated in the RISC complex, because the strand that forms the weakest base pair at its 5-end is incorporated into the RISC complex.
RNAi is only one of several strategies for mediating inhibition of gene expression using oligonucleotides, including the RNA complexes of this invention. These different strategies, that include RNase H mediated RNA cleavage, steric block RNA binding, DNAzyme or Ribozyme mediated RNA cleavage and siRNA approaches have been described in the literature together with the nature of selected chemically modified nucleotides that are compatible with biological activity [J. Kurreck, Eur. J. Biochem. 2003, 270, 1628].
The hydroxymethyl substituted monomers B-E of the invention have been incorporated into DNA strands, and therefore procedures for preparation of their phosphoramidite building blocks for automated DNA/RNA synthesis have been reported [K. D. Nielsen et al., Bioorg. Med. Chem. 1995, 3, 1493; H. Thrane et al., Tetrahedron 1995, 51, 10389; P. Nielsen et al., Bioorg. Med. Chem. 1995, 3, 19]. It is exclusively thymine monomers that have been incorporated into DNA strands. None of the hydroxymethyl substituted monomers have previously been incorporated into RNA strands.
In one report, one or two 2′-secouridines was incorporated into a DNA oligonucleotide and a positive effect on RNase H mediated RNA degradation was observed (Mangos M M, Min K L, Viazovkina E, Galarneau A, Elzagheid M I, Parniak M A, Damha M J., J Am Chem Soc. 2003 Jan. 22; 125(3):654-61).